With the advent of stricter governmental controls for engine emissions and increased concern to reduce weight of passenger vehicles, there arises a need for conserving the residual heat of exhaust gases of an internal combustion engine so that downstream equipment in a vehicle exhaust system may operate with higher efficiency and effectiveness to reduce the emission levels of the engine and conserve fuel. This need has become quite apparent to the automotive industry and is currently under intense development effort. Any solution to this problem must be simple, durable, and yet not introduce any additional problems.
Heat loss, experienced by the exhaust gases as they travel from the combustion zone through the exhaust passage of the engine block, can be considerable. Such heat loss is accomplished by conduction, convection and radiation. Minimizing heat loss within the exhaust passage is important for at least two principal reasons, (a) to maintain a high temperature of the exhaust gases therein to induce oxidation, and (b) to reduce the heat loss to the surrounding coolant in the block and head so as not to prematurely dissipate an unduly large number of heat units.
The prior art has approached such problems in principally three modes comprising: (1) use of cast-in-place type liners which have been either of the single metal layer or single refractory element design, or dual metal or refractory layers; (2) the use of insertable type liners which are added independently of the fabrication of the engine housing, such liners also being of the single layer heat resistant alloy metal design or double layer metal design or multiple layers of ceramic including air spaces or foamable paste therebetween; and (3) the use of applied coatings directly to the prefabricated engine housing passage walls, including asbestos and other ceramic materials. The disadvantage to employing cast-in-place type liners to date has been principally a lack of bonding; shrinkage and solidification of the cast metal around the liner has lead to localized poor bonding and/or separation which eventually provides for leaks and inadequate insulation. The principal disadvantage to the insertable type liner is that they insufficiently control heat transfer by not conforming closely to the wall of the exhaust passage resulting in a poorly trapped air space and a reduction in the insulating factor resulting from sealing difficulties. Coatings have proved disadvantageous because of their fragile nature which is particularly troublesome when the cast housing is subjected to post mechanical or chemical treatments tending to fracture or chip such coatings. Moreover, such coatings require multiple steps which result in increased manufacturing costs.